Wednesday 17 July 2013 06.37 EDT
First published on Wednesday 17 July 2013 06.37 EDT

Instant messaging app WhatsApp Messenger has more than 250m active users, making it bigger than Twitter.

Now the company has changed its iPhone app from a paid to a free download, while introducing the $0.99 annual subscription that it already charges on Android and other devices.

"We are updating our business model for new iPhone users going forward. As you know, we used to charge iPhone users a $.99 one time payment, while Android and other platforms had free service for the first year and paid $0.99 a year after that," explained WhatsApp in a blog post.

"From now on, we've simplified our business model so that all users on all platforms will enjoy their first year of WhatsApp service for free, and only pay $.99 per year after that."

Existing users of WhatsApp on iPhone won't have to pay the annual subscription, however: "WhatsApp will be free of charge for the rest of your life," promises the company, which also sets out grand ambitions for its life as a fully-freemium product.

"We feel that this model will allow us to become the communications service of the 21st century, and provide you the best way to stay in touch with your friends and family with no ads getting in the way."

WhatsApp has been at the forefront of a new wave of messaging apps that have been causing headaches for mobile operators' text messaging revenues, as well as existing social networks like Facebook and Twitter.

It launched in July 2009, and three years later is processing more than 8bn inbound and 12bn outbound messages.

In April this year, industry analyst Informa Telecoms and Media estimated that daily traffic from WhatsApp and other "OTT" [over-the-top] messaging services had already overtaken daily text-messaging traffic. The company predicted that by the end of 2013, 41bn OTT messages will be sent every day, compared to 19.5bn SMS texts.

WhatsApp has stuck doggedly to an ad-free model for its app, in contrast to some of its larger rivals in the social networking space.

"We're so bombarded with ads so much in our daily lives and we felt that smartphones aren't the place for that. Our phones are so intimately connected to us, to our lives. Putting advertising on a device like that is a bad idea. You don't want to be interrupted by ads when you're chatting with your loved ones."

WhatsApp's competitors are trying other ways to get users to spend money, including virtual stickers and social games.

New features are also being added with every new update to WhatsApp's app. Besides changing from paid to freemium, the latest version of the iPhone app includes support for Apple's iCloud to store backups of people's conversation history.

The company has also launched "developer hooks" to make it easier for other applications to connect to WhatsApp on iPhone.